In an automotive industry, it has recently become an urgent need to develop a car body structure capable of reducing an injury to a passenger at the time of a crash. It is possible to realize such a car body structure excellent in crash safety by absorbing impact energy at the time of the crash by structural members except a passenger section to minimize deformation of the passenger section, thereby securing survival space. That is, it is important that the structural members absorb the impact energy.
A main structural member absorbing the impact energy at a full-lap collision or an offset collision of an automobile is a front side member. A cross section of the front side member is closed by spot welding after stamping or the like. Generally, a buckling occurs in this front side member so as to absorb the impact energy. In order to improve the absorption of the impact energy, it is important to stabilize a buckling mode so as not to cause a bending or a fracture halfway.
The spot welding of the member has a problem that, unless a spot welding interval, a nugget diameter, and a welding condition are optimized to stabilize the buckling, a fracture occurs from a welded point during the buckling to cause an unstable buckling mode, resulting in reduction in the absorption of the impact energy.
In order to solve this type of problem, conditions realizing stable buckling without causing any fracture at a welded point have been conventionally studied by experimentally manufactured member under varied spot welding intervals and conducting a buckling test.
This method, however, requires trials and errors, that is, the experimental manufacture and the test are necessary for each automobile and each member. This has problems that the manufacture is costly and it takes a long time for designing.
As a method of estimating a fracture limit of a spot welded portion, various proposals have been conventionally made, and for example, Japanese Laid-open Patent Publication No. 2005-148053 (the undermentioned Patent Literature 1) describes a method in which a shear tensile test or a cross tensile test of a test piece having a spot welded portion is conducted to find, in advance, a relation of a ratio of a nugget diameter d to a width of the test piece and a stress concentration factor α, and fracture limit loads of the shear tensile test and the cross tensile test are estimated for a material having a given tensile strength, whereby a fracture limit load in a new test condition or in a spot welded portion of an actual member is estimated.
Further, Japanese Laid-open Patent Publication No. 2005-315854 (the undermentioned Patent Literature 2) describes a method in which a tensile test of a flange having a spot welded portion is conducted, moment efficiency γ is found in advance from a bending moment applied to an end portion of the spot welded portion and a full plastic moment Mp theoretically calculated from a sheet thickness, a sheet width, and a strength characteristic of a sample sheet, and from this moment efficiency γ and a full plastic moment Mp′ to a material having a given sheet thickness, sheet width, and strength characteristic, a fracture limit moment of a spot welded portion in the flange tensile test is estimated.
Further, Japanese Laid-open Patent Publication No. 2005-326401 (the undermentioned Patent Literature 3) describes a method in which based on a cross tensile test and/or a shear tensile test of a spot welded joint, a fracture strength parameter of a spot welded portion in cross tension and/or shear tension is calculated from all or one of a material strength, a sheet thickness, a spot-welding nugget diameter, a sheet width of the joint, and a rotation angle of the joint in the tensile test, the fracture strength parameter of each type of steel is stored, and the stored fracture strength parameter is introduced into a fracture prediction formula in which deformation of a periphery of the spot welding is modeled by a finite element method, and a fracture of the spot welded portion is determined.
Further, Japanese Laid-open Patent Publication No. 2007-304005 (the undermentioned Patent Literature 4) describes a method in which, based on a cross tensile test and/or a shear tensile test of a spot-welded joint, some or all of a sheet thickness, a spot-welding nugget diameter, a material strength of a base material, and a fracture elongation, and one or both of a joint welding interval and a joint length perpendicular to the welding interval are input to a computer, the computer calculates, from the input data, a rupture strain parameter of a spot welded portion in cross tension and/or shear tension, the rupture strain parameter of each type of steel is stored in a parameter storage, the rupture strain parameter stored in the parameter storage is introduced into a rupture prediction formula in which deformation of the periphery of the spot welding is modeled by a finite element method, and a rupture of the spot welded portion is determined, whereby a rupture of a spot welded portion of, for example, an automobile member is predicted in finite element method analysis on the computer.